The analysis of regulated gaseous chemical compounds from smoke stacks and other industrial processes is necessary for compliance with federal, state and/or local regulations in most industrialized nations of the world. Many apparatuses have been devised to measure these compounds. These apparatuses must provide for the specific gas measurements of interest while eliminating solid particles and prevent condensation of water and other vapors which are coresident with the gas or gases of interest.
Present dilution systems suffer three important disadvantages compared to the requirements of the measurement system. First, clogging of the sample flow control element due to ineffective removal of solid and viscous matter, causes changes in the flow rate of the sample to be diluted thereby necessitating maintenance of the apparatus on a more frequent basis then desired. Secondly, dependency on the consistency of the flow control elements, typically critical orifices and/or capillary tubes, for stability of the dilution ratio over time and associated operating temperatures and other operating conditions is required. But in fact, the flow throughput does change with temperature, pressure, gas density and contamination, resulting in errors in the gas concentration measurements. Finally, certain gases to be measured are indigenous in the atmosphere. In prior art devices, indigenous compounds such as carbon dioxide and carbon monoxide must be totally removed from the air to be used for the dilution. If not, their concentrations will be added to the concentration of the sample gas which significantly complicates the analytical measurements and often yields erroneous results. Having exact knowledge, by measurement, of the actual dilution ratio is the only practical means to relate the concentration of the compounds as measured in the diluted state to the concentrations as they existed before dilution. Determination of in-process concentration is necessary for the measurement to be compliant with legal requirements.
In the prior art devices, the operator is only able to compensate for changes in the dilution by calibration with gases of known concentration. This calibration can only be done infrequently, such as daily, otherwise it will reduce the percentage of available measurement data because the apparatus can only perform one measurement at a time. In other words, no measuring is being done when the device is being calibrated. As a result, the actual dilution ratio may change between the relatively long time intervals between calibration, resulting in inaccurate process concentrations.
As the requirements for controlling pollution from smoke stacks becomes more stringent, it is important that suitable monitoring equipment be provided to minimize the effect of particulate material on the filters to minimize the necessity of cleaning them. As a result, filtration systems, which can easily be obstructed by particulate material and liquid droplets in the gas stream, must be frequently removed increasing the down-time and cost of sampling. Also, appropriate compensation for dilution ratio fluctuations is not provided. The contaminated probes cannot be cleaned easily and returned to service.
One such probe is shown in commonly assigned U.S. Pat. No. 4,974,455 to McGowan et al. This probe includes a filter which must block all entrained particulate material in the sample, requiring frequent replacement of the filter. Also, no provision is provided for compensating for the variation in flow rate do to clogging of the filter.
Other prior art dilution probes wherein the sample air from the stack passes through a filter are EPM Model 797 Dilution Probe sold by EPM Environmental, Inc. of Mount Prospect, Ill., U.S.A. and Holland Oxydan Dilution Probe manufactured by Oxydan AS of Stilling, Denmark.